Recently, in the field of radio communication, the MIMO (Multiple-Input and Multiple-Output) technique has been utilized, which realizes an improvement in the speed and quality of signal transmission by executing transmission and reception using multiple antennas on each of the radio transmission device side and on the radio reception device side. In order to further improve signal transmission speed and further reduce interference, a massive-MIMO transmission scheme has been considered which uses a large number of antenna elements (e.g., 100 elements or more) in a high frequency band (e.g., 10 GHz or more) that enables antenna miniaturization and in which a wide bandwidth can be secured (e.g., Patent Document 1). FIG. 1 schematically shows ordinary MIMO and massive-MIMO.
With massive-MIMO, advanced beamforming (BF) can be implemented that uses a greater number of antenna elements in comparison to conventional MIMO. Beamforming is a technology that involves controlling the directivity and/or shape of a beam (e.g., a transmission beam corresponding to a transmission antenna, or a reception beam corresponding to a reception antenna) by controlling, for multiple antennas, the phase and amplitude of the respective signals. FIG. 2 schematically shows a beam to which beamforming is not applied, and FIG. 3 schematically shows a beam to which beamforming is applied.
With MIMO, the phase and amplitude can be controlled for each antenna element, and therefore, the flexibility of beam control increases with an increase in the number of antenna elements that are used, allowing advanced control.
With massive-MIMO, beamforming of different modes are assumed. The following exemplifies an overview of each of analog beamforming (ABF), digital beamforming (DBF), digital precoding (DP) full-digital massive-MIMO, and hybrid beamforming (HBF).
With analog beamforming, beams are formed using a variable phase shifter, which is analog circuitry. This analog circuitry operates at a radio frequency (RF) or an intermediate frequency (IF). Beams formed by analog beamforming are common for all frequency bands. These beams are generally directed to the direction of arrival of a radio signal. With analog beamforming, there is a possibility for a characteristic deterioration to occur as a result of inter-beam interference. On the other hand, analog beamforming can be implemented with a simpler circuit configuration compared to digital beamforming since it is possible to reduce the numbers of up-converters, down-converters and baseband processing circuits.
Digital beamforming is beamforming technique for executing beamforming by a digital signal processing, and beams are formed not by a variable phase shifter but by baseband processing circuitry. Beams formed by digital beamforming are common for all frequency bands. With digital beamforming, there is a possibility that inter-beam interference can be reduced by performing digital signal processing. On the other hand, the circuit configuration is more complex with digital beamforming since it is not possible for the numbers of up-converters, down-converters and baseband processing circuits to be reduced.
With digital precoding full-digital massive-MIMO (full-digital array), it is possible to optimize channels by forming beams separately for different frequencies. With digital precoding full-digital massive-MIMO, there is a possibility that inter-beam interference can be reduced by performing digital signal processing. In controlling digital precoding full-digital massive-MIMO, channel state information (CSI) for each frequency is required. The circuit size of digital precoding full-digital massive-MIMO is the same as that of digital beamforming.
Hybrid beamforming is a combination of analog beamforming and digital precoding. With hybrid beamforming, analog beamforming is used to form beams. Therefore, it is possible to reduce circuit size. The inter-beam interference that occurs as a result of analog beamforming can be reduced by using digital precoding. With hybrid beamforming, similar characteristics as those of digital precoding may be realized by optimizing the numbers of up-converters, down-converters and baseband processing circuits. FIG. 4 and FIG. 5 show example configurations of hybrid beamformers used for hybrid beamforming. FIG. 4 shows a full-array type configuration, and FIG. 5 shows a sub-array type configuration. In either configuration, the number of chains for RF circuits and baseband processing circuits is L, and the number of transmission antenna elements is NT.